Method for infrared spectroscopy

ABSTRACT

A process and apparatus for obtaining nongaseous pyrolyzate samples of materials for infrared analysis is described. The collecting devices or cells for nongaseous pyrolyzates which at ordinary temperature may be liquid or partially solid are provided with a heat ribbon having a depression to take the pyrolysis sample and a cover with a hermetically sealable opening taking a pyrolyzate plate which may be an attenuated total reflection plate, one side being exposed to the pyrolysis vapors on the inside and the other coolable. The heater is connectable to a power source and the cell to a source of vacuum with a valve which can be shut after pyrolysis is complete to maintain the vacuum.

United States Patent METHOD FOR INFRARED SPECTROSCOPY 2 Claims, 3Drawing Figs.

[1.8. CI. 356/38, 250/833, 250/218, 356/51, 356/244, 356/246, 1 18/49.1Int. Cl. G0ln l/00, G01n33/28,G01n1/10 Field of Search 356136- PrimaryExaminer-Ronald L. Wibert Assistant ExaminerWarren A. Sklar Attorneys-Robert Ames Norton and Joseph Levinson ABSTRACT: A process and apparatusfor obtaining nongaseous pyrolyzate samples of materials for infraredanalysis is described. The collecting devices or cells for nongaseouspyrolyzates which at ordinary temperature may be liquid or partiallysolid are provided with a heat ribbon having a depression to take thepyrolysis sample and a cover with a hermetically scalable opening takinga pyrolyzate plate which may be an attenuated total reflection plate,one side being exposed to the pyrolysis vapors on the inside and theother coolable. The heater is connectable to a power source and the cellto a source of vacuum with a valve which can be shut after pyrolysis iscomplete to maintain the vacuum.

PATENTEUJUH] 5mm 34,584,957

sum 1 BF 2 INVENTORS. STANLEY E. POLCHLOPEK ROBERT L. HA RR/S ATTORNEYSHEET 2 BF 2 STANLEY E. POLCHLOPEK ROBERT L. HARRIS ATTORNEY METHOD FORINFRARED SPECTROSCOPY RELATED APPLICATIONS This application is adivision of our earlier application Ser. No. 453,030, filed May 4, 1965,and now U.S. Pat. No. 3,508,836.

BACKGROUND OF THE INVENTION In the past it has often been desirable topyrolyze materials and collecting samples for infrared or other spectralanalysis.

.The problem is most common for infrared analysis and the description inthe specification will be in terms of this purpose. However, theinstruments are useful for other types of spectral investigation. One ofthe reasons why pyrolysis of samples is desirable is because manysubstances such as, for example, rubber with fillers and other polymersdo not lend themselves to direct spectral analysis either bytransmission or attenuated total reflection including both single andmultiple reflection, which in the remainder of this specification willbe abbreviated in its customary manner ATR. The materials are oftendifficult to mount for spectral analysis and the presence of fillers cancompletely mask the spectral analysis of the organic materialsthemselves. Accordingly, pyrolysis has been used to decompose thematerial or depolymerize the polymers and collect the pyrolyzate inliquid form. In general most fillers are not decomposed and, therefore,the resulting spectrum is free from their contribution. In the past thishas been effected manually, for example, by placing a small amount ofthe material in a test tube, heating the material and collecting thepyrolyzate either as gas in the upper part of the test tube or as liquiddrops on the walls of the upper part of the test tube.

A number of problems have arisen in the manual pyrolysis of whichseveral illustrative ones will be mentioned. Often the materials duringpyrolysis are reactive with atmospheric gases, particularly oxygen, andit is extremely difficult or impractical to exclude the presence of suchan atmosphere. Another problem is that the pyrolyzate has to be on aparticular-type of plate for ATR spectral analysis and when ordinaryinfrared analysis is used, the materials of a test tube may presentproblems of lack of transparency for the infrared which cuts outportions of the spectrum which are sometimes of importance. Often thesample derived from a pyrolysis in a test tube is also insufficient involume for transfer to an ATR plate or crystal.

SUMMARY OF THE INVENTION The present invention solves the problem ofproducing automatically nongaseous samples of the products of pyrolysisin a form suitable for spectral analytic investigation. Essentially thepresent invention provides a device with a suitable heating elementwhich can be plugged into a power supply and on which heating element asmall amount of the material to be pyrolyzed is placed. The heatingelement is surrounded by a chamber or container having a portion,usually the top portion, provided with a removable element on whichliquid pyrolyzates can be collected. The chamber is provided with aconnection to a source of diminished pressure such as a vacuum pump oraspirator and with a valve which can be open to exhaust or partiallyexhaust air from the chamber and then maintain the low pressure byclosing the valve. The chamber is then plugged in with the power supplyand the heating element heated for a predetermined period and at apredetermined temperature resulting in pyrolysis of the sample andcollection of the nongaseous pyrolyzate.

The collecting element may be a disc of suitable material having thecorrect radiation transmission which is strongly cooled and on which thepyrolyzate collects in drops. The element is then removed, if necessary,provided with a cover in a manner similar to the cover glass of amicroscope slide, and is then examined spectrally, either by aspectrophotometer or by ATR. In the latter case, of course, the elementmust be in the form of a suitably shaped ATR crystal.

Although some kind of a power supply is needed into which the cells canbe plugged, the particular design of power supply forms no part of thepresent invention so long as it can furnish electric power for theheating element of the proper voltage and for the proper time. In thespecific description of a particular instrument below, the power supplyis provided with a conventional timer which turns off the power supplyafter the predetermined time period and with switches for selectingseveral different voltages for the heating element, for example, threedifferent voltages. The design of switches and timer are completelyconventional and so the present invention is not to be considered aslimited to any particular type of power supply although the devices ofthe present invention require some power supply for operation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of the top of atypical power supply;

FIG. 2 is a vertical section through a liquid pyrolyzate collector forspectrophotometric use; and

FIG. 3 is a plan view of a modified liquid pyrolyzate collector forcollecting the pyrolyzate on a normal ATR crystal or plate.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, the powersupply is in a box 1 provided with electric wires 2, a conventionalsettable timer 3, a starting switch 4, three voltage selecting switches5, 6 and 7 and two jacks 8. As the elements in the power supply box arecompletely conventional they are not shown. When the starting switch 4is depressed after one of the switches 5, 6 or 7 has been actuated for avoltage selection, the selected voltage appears on jacks 8 until thetimer 3 has moved through its set time interval at which time, as isnormal with timers, the starting switch is thrown back into its originalopen position.

As has been stated, the exact design of the power supply forms no partof the present invention. However, typical values for the power supplyare three voltages which, with a standard resistance heating element,produce 450 C., 900 C., or 1350 C. This choice of temperatures issuitable for most pyrolysis but, of course, where a differenttemperature is needed in some special case, a modified power supply isprovided with a suitable voltage output.

Turning to FIG. 2 which illustrates a liquid pyrolyzate collector, thecollector has two pins 9 which plug into the jacks 8. These pins passthrough a suitable insulating base 12 and connect to the ends of aribbon heating element 10 provided with a small depression 1 l toreceive a sample of the material to be pyrolyzed. The heating element isin a chamber 13 sealed from the insulating base 12 by a suitable gasket14 in the form of an O-ring. The top of the collecting chamber isprovided with a flanged opening in which a disc 20 of suitabletransparent material is mounted. For infrared use this may be a suitablesalt disc. The chamber is on a common base 28 on which there is mounteda cover 19 which communicates with the chamber 13 through the channel15. This cover which forms a second chamber is provided with a suitablevacuum valve 16 actuated by a handle 17 and provided with a serratedconnecting end 18 projecting beyondthe cover 19. The serrated projectingchannel is suitable for attachment to elastic tubing from a source ofdiminished pressure (not shown). Of course, if the source utilizes ahose with a different coupling the element 18 must be provided with theproper design of coupling. These features are conventional and,therefore, only the rubber hose form is shown by way of illustration.

A block 22 of material preferably at a very low temperature, forexample, a metal block which has been cooled in liquid nitrogen or athermoelectric cooler-is placed on the disc 20 cooling it down to thedesired low temperature. Before the disc 20 has been mounted a sample ofthe material to be pyrolyzed is placed in depression 11. After pluggingin the collector in the jacks 8 of the power supply, the switch 4 isclosed and the element 10 is heated up to the temperature selected bythe particular switch 5, 6 or 7 which is depressed and for the timeduring which the timer is set to operate. Typically the latter may beabout seconds and a typical dimension of the disc is a circular disc ofmm, diameter. The small sample of material in the depression 11 ispyrolyzed and the condensable vapors are condensed on the cold disc 20.Before the operation is started, the lever 17 is thrown to the openposition and the atmosphere in the chamber 13 is reduced to the desiredsubatmospheric pressure whereupon the valve 16 is closed.

After collection of the liquid pyrolyzate on the cold disc 20 the discis removed and, if desired, a cover plate of suitable transparentmaterial placed over the pyrolyzate droplets. The disc then can beexamined by placing in the holder of a conventional infraredspectrophotometer. Typical models of such spectrophotometers haveholders which take a 25 mm, disc which is the reason for the choice ofdevice size as described above. The liquid film of pyrolyzate betweenthe cover and the disc is then examined spectroscopically and in thecase of a recording spectrophotometer, a record of the infraredabsorption spectrum produced.

When the sample was of a polystyrene plastic with a filler the spectrumshows no interference by reason of the fillers. For such a plastic thetemperature of 900 C., for 6 seconds is suitable.

Successive pyrolyzate samples can be obtained by cleaning off the heater10 and inserting a new collector disc without unplugging the collectorfrom the power supply. However, in many cases it is more convenient tohave a number of collector chambers available so that a number ofsamples can be examined quickly and in such a case the cells can beunplugged and disconnected from the source of lowered pressure.

HO. 3 illustrates in plan view a cell of the same general design as HO.2 but provided with an elongated opening 21 with a gasket 23 to take atypical ATR plate. The same elements bear the same reference numerals asin FIG. 2. When the collecting cell with mounted ATR plate is operatedeither at room temperature or cooled as described above, a liquidpyrolyzate example is obtained which is then spectrally analyzed by ATR.As only a thin film of the liquid is needed for such an examination theATR modification presents advantages where the absorption bands of theliquid film do not have to be at all critical in thickness. This issometimes of advantage with certain materials.

A typical example of operation of the collector of F IG. 3 involves thepyrolysis of an epoxy resin. in this case the temperature of 900 C. isused but the time is advantageously set for 10 seconds. An excellent ATRanalysis is obtained which would be impossible by ordinary ATR meanssince too hard and rough a surface is presented by the unpyrolyzedresin. in the chamber referred to above there is a small amount of watervapor produced by the pyrolysis of the epoxy resin and the ATR infraredspectrum, therefore, showed absorption bands for water vapor. As theposition of these absorption bands is well known they do not create anyconfusion in the interpretation of the spectrum produced.

We claim:

1. A process of infrared spectral analysis of nongaseous phasepyrolyzates comprising, in combination,

a. heating at subatmospheric pressure a substance capable of pyrolysisto form vapors condensable to the nongaseous phase and relativelynonvolatile at room temperature, the heating being to a temperaturesufficiently high to pyrolyze the material at the subatmosphericpressure to produce pyrolysis vapors,

. exposing one side of an infrared transmitting plate to the pyrolysisvapors, and maintaining the other side of the plate out of contact withthe pyrolysis vapors,

c. cooling said other side of the plate to a temperature at whichcondensable pyrolysis vapors condense to the nongaseous stage, whereby alayer, of the nongaseous pyrolyzate is formed on said one side of theplate, and

d. analyzing the pyrolyzate layer on the plate by infrared spectralanalysis. I 2. process according to claim 1 in which the plate is an ATRsubstrate and wherein the step of analyzing comprises analysis byattenuated total techniques.

1. A process of infrared spectral analysis of nongaseous phasepyrolyzates comprising, in combination, a. heating at subatmosphericpressure a substance capable of pyrolysis to form vapors condensable tothe nongaseous phase and relatively nonvolatile at room temperature, theheating being to a temperature sufficiently high to pyrolyze thematerial at the subatmospheric pressure to produce pyrolysis vapors, b.exposing one side of an infrared transmitting plate to the pyrolysisvapors, and maintaining the other side of the plate out of contact withthe pyrolysis vapors, c. cooling said other side of the plate to atemperature at which condensable pyrolysis vapors condense to thenongaseous stage, whereby a layer of the nongaseous pyrolyzate is formedon said one side of the plate, and d. analyzing the pyrolyzate layer onthe plate by infrared spectral analysis.
 2. A process according to claim1 in which the plate is an ATR substrate and wherein the step ofanalyzing comprises analysis by attenuated total techniques.